Method of producing molds for injection molding

ABSTRACT

Molds for injection molding are produced by providing a primary mold core, coating the primary mold core with a water-resistant coating layer, applying on the water-resistant coating layer a separating layer, spraying a metal shell on the separating layer, arranging a rear filling on the shell so as to form a tool part, and supplying water to the water-soluble layer so as to remove the tool part.

CROSS REFERENCE TO A RELATED

This application is a continuation-in-part of patent application Ser.No. 573,566, filed Jan. 25, 1984, now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to a method of production of molds forinjection molding, particularly of tools for injection molding ofplastics.

Known methods of production of tools for injection molding of plasticsinclude the utilization of high-grade tools which produce molds fromfine-steel blocks by material removing operations. The quality andaccuracy of the injection molding workpieces depend primarily on thehardness of the workpiece material. The material-removing working iscorrespondingly tedious and cost-consuming, so that in the event ofcomplicated molds for production of tools high investment is neededwhich is possible only for high numbers to be produced. When theproduction of such tools is planned, it is necessary to be sure inadvance that the workpiece to be produced has a design which is optimalfor the sale and the use. Changes in the tools are almost not possibleor can be done in a small range, for example with the aid ofelectroerosion, which is also a tedious and expensive process. Changesof a greater scale can be accomplished only by production of a new tool.

A tool of a less hard material is less expensive to manufacture;however, it makes possible a lower number of injection and producesworkpieces of lower accuracy. In the sense of costs, such less expensivetools are profitable for objects which do not have exact mass and whosenumber are relatively small, such as for example toys. Such simple toolsfor correspondingly simple workpieces are cast for example of zincalloys. They are also formed as hard nickel mold partsgalvanoplastically from a primary mold core. The tools cast on zincbasis have the disadvantage that zinc has a tendency to bubbleformation. These simple tools can withstand at best 5000-10,000manufacturing cycles, whereas fine-steel tools of hard-grade alloyedglass-hard steel can carry out 1,000,000 injections.

When before the production of high-grade tools the workpiece is testedin such a manner that, first, they are produced in a testing processwith simpler tools, additional costs are involved for testing tools, onthe one hand, and there is a risk that the research will be of no valuebecause of the insufficient and unreliable accuracy, on the other hand.The high cost of production of high-grade injection tools in accordancewith the existing methods considerably blocks the expansion of theinjection molding technique. Each production of a new model involves ahigh investment risk. In the case when a model is to be introduced intoa market, high tooling costs play a negative tool. As soon as the marketrequires changes, new investments must be made.

These conditions are typical both for the synthetic plastics industryand especially for the tire industry which is in a phase of continuousinnovation, since improved profiles for winter tires are searched for asa replacement for spoked tires. During test research the profile isoften cut from a smooth protector, since the production of small seriesin accordance with injection-molding methods is very expensive and forone individual tire four or more segment-shaped tools are needed to beassembled on a common tool. Changes in molds required for special tires,such as tires for street and ground construction, for farming tractorsand for cross-country vehicles involve, in relation to the number ofpieces, too high costs.

Similar cost difficulties are encountered in the case of injectionmolding of metal, so here the tools are not so expensive, but they arenot durable.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide amethod of production of molds for injection molding which avoid thedisadvantages of the prior art.

More particularly, it is an object of the present invention to provide amethod of production of molds for injection molding which makes possiblea faster and very cost-favorable production, on the one hand, andproduces the molds which are as high-grade and as hard in their surfacesas the qualitatively best tools produced from fine-grade blocks inmaterial removing operations.

In keeping with these objects and with others which will become apparenthereinafter, one feature of the present invention resides in a method ofproduction of molds for injection molding in accordance with which aprimary mold core is provided, a water-resistant coating layer isapplied on the primary core, a water-soluble layer is applied on thewater-resistant layer, then a fine-steel or a suitable metal or ceramicin melted condition is applied on the water-soluble layer, and afterforming the shell with a rear filling of non-yieldable material so thatthey together form a mold part, water is brought in contact with thewater-soluble layer to separate the mold part from the primary moldcore.

When the method is performed in accordance with the present invention asdefined hereinabove, it eliminates the disadvantages of the prior art.

The novel features which are considered characteristic for the inventionare set forth in particular in the appended claims. The inventionitself, however, both as to its construction and its method ofoperation, together with additional objects and advantages thereof, willbe best understood from the following description of specificembodiments when read in connection with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a front view of a synthetic plastic shell (prototype) formedby a mold produced in accordance with the present invention;

FIG. 2 is a view showing the synthetic plastic shell of FIG. 1 in a rearview;

FIG. 3 is a view showing an extruder-spaced part of the mold, partiallyin section X--X in FIG. 1, which has a fine-steel mold shell with anepoxy resin-quartz sand rear filling in a steel housing with a steelcover;

FIG. 4 is a view showing an extruder-close part of the tool, partiallyin section X--X in FIG. 2, including a fine-steel mold shell with anepoxy-resin quartz sand rear filling, arranged in a steel housing;

FIG. 5 is a view showing the mold parts of FIGS. 3 and 4 assembled intoa common tool on an extruder, in section X--X of FIGS. 3 and 4;

FIG. 6 is a view showing the extruder-spaced mold part of FIG. 3 formedon a primary mold core of gypsum with the shape corresponding to theshape of the surface of the shell of FIG. 1, wherein a steel housingwith a cover surrounds this mold part and accommodates a rear fillingcomposed of epoxy resin and quartz sand;

FIG. 7 is a view showing a fragment A of FIG. 6 on a considerablyenlarged scale;

FIG. 8 is a view showing the steel housing with cover, both inperspective; and

FIG. 9 is a view showing both mold parts with rear filling of metalcasting and with respectively simplified housing cover.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Prototypes of the workpiece to be produced are shown in FIGS. 1 and 2and identified with reference numerals 1 and 2. A primary mold core isidentified with reference numeral 3. A support 4 is provided for theprimary mold core 3 and a water-resistant coating layer 5 is arranged onthe latter. A water-soluble separating layer 6 follow thewater-resistant coating layer 5, and a fine-steel layer 7 is arranged onthe water-soluble separating layer 6. A housing 8 surrounds thefine-steel layer 7, and a rear filling is provided on the latter. Thehousing 8 has a cover 10 with a cover edge 11 with a conical incline 12.An opening 13 is provided in the center of the cover 10.

Reference numeral 14 identified a tool part which is spaced from theexterior, whereas reference numeral 15 identified a tool part which islocated close to the exterior. The tool parts have a metallic rearfilling 16. Screws 17 are provided for mounting of the edgeless cover10. Reference numeral 18 identified a copper layer on the fine-steellayer 7.

In accordance with the inventive method, first of all the primary moldcore 3 of gypsum shown in FIG. 6 is produced with the aid of theprototypes 1 and 2 shown in FIGS. 1 and 2. The primary mold core 3 shownin FIG. 6 corresponds in its surface design to the prototype 1. The notshown primary mold core corresponding to the prototype 2 of FIG. 2 iscast in an analogous manner from gypsum. The primary mold cores can alsobe formed of other materials, as long as they can be subjected totemperatures around 100° C. without deformation, such as for examplewood, synthetic plastic, and the like.

Each primary mold core 3 is arranged on a geometrically flat support 4.The water-resistant cover layer 5 is applied on each primary mold core 3with the aid of a conventional paint spray gun to a layer thickness of15-30 μm. The water-resistant coatng layer can be formed from a solutionincluding the following components:

    ______________________________________                                        latex           38%        by weight                                          water           25%        by weight                                          finely ground quartz                                                                          25%        by weight                                          acrylic resin   8%         by weight                                          hydrosol        3%         by weight                                          alkyde lacquer  1%         by weight                                          ______________________________________                                    

The latex can be VINNAPAS (polyvinylacetate in aqueous solution) byWacker-Chemie GmbH (German trade mark);

The acrylic resin can be ACRYLSAEURE (acrylic acid) METACRYLSAEURE(methacrylic acid), ACRONAL (polyacrylicester) by BASF AG (all Germantrade marks);

The hydrosol can be HYDROSOL by Huels AG, containing acrylic acid inaqueous dispersion;

The alkyde lacquer can be ALCYDHARZLACK (alkyd lacquer) by Bayer AG, orALKYDAL (polyisobutylene) by BASF AG (all German trade marks), orcomposition of linseed oil with polyalcohol.

After drying of the coating layers 5, a water-soluble separating layer 6is sprayed on the coating layer 5 so as to have substantially the samelayer thickness as the layer thickness of the layer 5. In many cases itis sufficient to have a total layer thickness of 30 μm for both layers.The watersoluble layer 6 can be produced from a solution including thefollowing components:

    ______________________________________                                        water            38%        by weight                                         finely ground quartz                                                                           36%        by weight                                         sugar dissolved in water                                                                       15%        by weight                                         adhesive         8%         by weight                                         metal powder     3%         by weight                                         ______________________________________                                    

Sugar dissolved in water can contain water 50% by weight and sugar 50%by weight.

In the composition the adhesive can be a wallpaper glue, which forexample has a tradename METHYLAN by Henkel GmbH, Duesseldorf. Itsessential ingredient is methylcellulose as water-soluble celluloseether.

Metal powder can be nickel metal powder.

As to the weight of water in the exemplary composition set forth above,this water is added water and does not include water from othercomponents. The water which is used in the above composition is asoftened water, as interpreted in "Encylopedia Britannica," Vol. 19, p.651.

The above mentioned specific solution used for the water-resistant layer5 has also the purpose of forming a water-resistant film on the surfaceof the primary mold core 3, which is also temperature-resistant. Thesolution used for forming the water-soluble layer 6 on thewater-resistant layer 5 produces a water-soluble film which under theaction of impacts and heat of the metal spraying jets does not produceany connection with the water-resistant layer 5 and remains chemicallyand mechanically separate therefrom. In the case of the primary moldcore 3 of gypsum, wood, synthetic plastic and the like, with which notemperatures over approximately 100° C. takes place, the samewater-resistant layer 5 can be used for spraying of many fine-steelmolds.

The above presented compositions of the layers 5 and 6 are given just asoptimal examples. Within the basic idea of the invention many suitablecompositions can be provided for these layers. It is noted that thebinding and releasing primers which are used in the present inventionare commercially available, for example from the inventor at Rheinbrohl,FRG.

The drying of the layers 5 and 6 can, if desired, be accelerated byheating. For the purpose of providing an especially smooth surface ofthe primary mold core, the mold cores can be carefully ground.Precautions must be taken that the separating layer 6 is not removedduring this grinding.

Then the fine-steel melted with the metal spray gun is applied onto bothprimary mold cores 3. During spraying it must be taken care of that nooverheating of the fine-steel layer 7 takes place. The spraying distancebetween the spray gun and primary mold cores must be at leastsubstantially between 30 and 50 cm, and the spraying takes place inseveral portions, so that after each spraying portion a short pause isprovided so as to enable the sprayed fine steel to be cooled.

At temperatures over substantially 130° C. the separating layer 6 isreleased from the fine-steel layer 7 so that pressure air or pressuregas can penetrate between the primary mold cores and fine steel layer.The fine steel layer leaks from the primary mold core and after thisbecomes non-usable. The high content of carbohydrates (sugar disolved inwater) serves here as a warning signal: at 100° C. the liquid sugarburns with formation of smoke. The operator of the spray gun thenrecognizes that he must provide a short pause so as to allow cooling ofthe fine-steel layer.

For providing the fine-steel layer 7 with a thickness of approximately 1mm, 15-20 individual layers are needed. For the utilization of thefine-steel layers 7 as mold parts of a tool for injection molding ofplastics, such thick shells are not needed. It is sufficient to haveconsiderable smaller thicknesses, approximately between 0.1 and 0.5 mm.Then, in connection with the tool, the fine-steel layer 7 has nosupporting functions, as will be explained below. Decisive for theutilization of the fine-steel layers 7 as a mold for injection moldingis in the first place the hardness of the material. In contrast to otherdeformation principles, the material hardness in the present inventionplays no role: the hardest chromium steels can be flame sprayed withoutdifficulties.

In the case of application of the fine steel in an oxygen-free spraycabin in accordance with DE GM No. 8,225,728 (later OS No. 3,233,925),the fine-steel layers or mold shells 7 formed on the primary mold cores3 are sinter-free and have a similar structure to workpieces produced bycasting or materialremoving cutting. Then it can be polishedrespectively. In the case of spraying of fine steel, particularly ofCr-V-steel or Mo-Al-steel in a spraying cabin filled with nitrogen, anadditional nitro hardening automatically takes place.

After cooling of the fine-steel mold shells 7 lying on the primary moldcores 3, each mold shell 7 is surrounded by an upwardly open ordownwardly open housing 8, and each housing 8 is filled to its edge witha mixture of epoxy resin and quartz sand forming the rear filling 9. Thecover 10 is placed on the housing 8 and extends with its cover edge 11into the housing 8. The cover edge 11 has a conical shell-shaping line12. The cover 10 is provided in its center with an opening 13. It ispressed on the housing 8 so that the opoxy resin-quartz sand mixturefirmly lies all over on the fine-steel mold shell 7. The cover edge 11is so dimensioned that it can be inserted into the housing 8 only withdifficulties, and therefore no safety measures are needed againstcarelessness, casting or falling out. The housing 8 and the cover 10 areshown in FIG. 8.

The rear filling 9 can be cmposed of a mixture of epoxy resin-ironpowder, epoxy resin/glass powder (or small glass balls). It must benon-elastic and non-yieldable and capable of taking high forces (sprayforce, on the one hand; and holding force, on the other hand) withoutyielding.

After introducing the rear filling 9, water is supplied between theprimar mold core 3 and the fine-steel shell 7. The water-solubleseparating layer 6 is dissolved, and the tool 14, 15 can be separatedfrom the primary mold cover.

A feed opening B provided in the tool part 15 connects a spray unit E ofan extruder with the tool 14, 15 as a whole. The tool part 14 which isspaced from the extruder is subjected to the action of the closing forcewhich corresponds to the spraying force acting upon the tool part closeto the extruder. The tool 14, 15 as a whole is pressed together by thetwo forces. The forces are very high, depending upon the machine sizeand the workpiece size; they can amount to between 10 and 5000 Mp. Sincethese forces act on the cover 10, each eventaul yielding of the rearfillings 9 is abosrbed by the covers in that the cover edges 11 aredisplaced inwardly into the rear filling 9 depending upon the respectiveyielding.

Absolute non-yieldability of the rear fillings is obtained when eachrear filling is composed of a low-melting metal which is cast in moltencondition in the housing 8. Before this it must be provided that thefine-steel shell 7 is brought to a temperature which is close to themelting point of the metal for the rear filling, since the fine-steelshell otherwise can be warped. The primary mold core 3 must be producedof respective heat-resistant material, for example of copper, with amelting point of 1080° C., whereas the rear filling can be produced ofzinc with a melting point of 420° C. or bronze with a melting point of700° C.

During heating of the primary mold core 3 and the fine-steel shell 7,the separating layer 6 and the coating layer 5 are burnt, so that theseparation of the workpiece part 14, 15 during filling of the metallicrear filling 16 takes place. Since the metallic rear filling 15 iscompletely non-yieldable, the cover 10 can act on the closing forceswithout abutting of the edge and collar on the rear filling, and can beheld by screws 17 and the like, as can be seen in FIG. 9.

The metallic rear filling is formed by metal extrusion molding forfurther production of automobile tires. The extrusion molding isperformed with temperatures which are not compatible with a primary moldcore of gypsum, synthetic plastic. etc. For the production of tools forthe tire fabrication is carried out in segments, and these segment toolsare assembled to an annular final tool.

When it is desirable to obtain a fast and uniform heat withdrawal fromthe fine-steel shell 7, it is advantageous after spraying and cooling ofthe fine-steel shells 7, to spray on it a copper layer 18.

It will be understood that each of the elements described above, or twoor more together, may also find a useful application in other types ofconstructions differing from the types described above.

While the invention has been illustrated and described as embodied in amethod of producing of molds for injection molding, particularly oftools for injection molding of synthetic plastics, it is not intended tobe limited to the details shown, since various modifications andstructural changes may be made without departing in any way from thespirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can, by applying current knowledge,readily adapt it for various applications without omitting featuresthat, from the standpoint of prior art, fairly constitute essentialcharacteristics of the generic or specific aspects of this invention.

What is claimed as new and desired to be protected by Letters Patent isset forth in the appended claims.

I claim:
 1. A method of producing molds for injection molding,particularly for tools for injection molding of synthetic plastics, themethod comprising the steps of providing a primary mold core; sprayingon a layer which forms a mold shell; arranging a rear filling so thatthe mold shell and the rear filling together form a mold part; providingbefore spraying, between the mold core and the mold shell-forming layer,two separate layers by coating the primary mold core with aheat-resistant and water-resistant coating layer and subsequentlyapplying on the coating layer a heat-resistant and water-solubleseparating layer; and supplying water to the water-soluble separatinglayer so as to release the mold part from the primary mold core.
 2. Amethod as defined in claim 1, wherein said providing step includesproviding the primary mold core of a fine steel.
 3. A method as definedin claim 1, wherein said providing step includes providing the primarymold core of a metal.
 4. A method as defined in claim 1, wherein saidproviding step includes providing the primary mold core of a ceramic. 5.A method as defined in claim 1, wherein said coating step includescoating of the primary mold core with a water solution of hydrosols,latexes, acrylic resin, finely ground quartz and alkyde lacquer.
 6. Amethod as defined in claim 1, wherein said applying step includesapplying a water solution of adhesive, carbohydrate, finely groundquartz and metal powder.
 7. A method as defined in claim 1, wherein saidapplying step includes applying said water-soluble separating layerafter drying of said water-insoluble coating layer.
 8. A method asdefined in claim 1, wherein said coating step includes coating of theprimary mold core with a water-resistant coating layer solution havingthe following composition:

    ______________________________________                                        latex           38%        by weight                                          water           25%        by weight                                          finely ground quartz                                                                          25%        by weight                                          acrylic resin   8%         by weight                                          hydrosol        3%         by weight                                          alkyde lacquer  1%         by weight.                                         ______________________________________                                    


9. A method as defined in claim 1, wherein said applying step includesapplying a water-soluble separating layer solution having the followingcomposition:

    ______________________________________                                        softened water   38%        by weight                                         finely ground quartz                                                                           36%        by weight                                         sugar/dissolved in water                                                                       15%        by weight                                         adhesive         8%         by weight                                         metal powder (nickel)                                                                          3%         by weight.                                        ______________________________________                                    


10. A method as defined in claim 1, wherein said arranging step includesarranging the rear filling composed of quartz sand.
 11. A method asdefined in claim 1, wherein said arranging step includes arranging therear filling composed of metal powder.
 12. A method as defined in claim1, wherein said arranging step includes arranging the rear fillingcomposed of epoxy resin.
 13. A method as defined in claim 1, whereinsaid arranging step includes arranging the rear filling composed of acast metal.
 14. A method as defined in claim 1; and further comprisingthe step of pressing a housing surrounding the primary mold core and themold shell, said arranging step including filling of the rear fillinginto the housing.
 15. A method as defined in claim 14; and furthercomprising the step of using a cover having a conical shell-shaped edgewhich engages the housing at all sides and abuts against the rearfilling.
 16. A method as defined in claim 1, and further comprising thestep of applying a copper layer on the mold shell arranged on theprimary mold core.